The present invention relates to a road condition discriminating system for discriminating the condition of the road on which a vehicle is running, in response to a vehicle speed signal generated by a vehicle speed sensor mounted on the vehicle.
In recent years, more and more electronic control devices have been incorporated into the chassis of automotive vehicles. A skid control device, a vehicle height control device, and a suspension control device are a few examples. In addition, such means as a power steering control device, a constant speed running control device and a transmission control device are being widely used.
The feature that these devices have in common is that a vehicle speed signal is used by some for the purpose of control. Thus, the use of a vehicle speed sensor is now becoming more common.
Where vehicle speed is detected by a wheel speed sensor mounted on a wheel, changes in wheel rotations are directly converted into signals.
Experiments show that the wheel speed V.sub.W irregularly changes on a rough road as shown in FIG. 1a so that the differentiated value of the wheel speed V.sub.W, that is, the wheel acceleration/deceleration V.sub.W in FIG. 1b, is subjected to irregular fluctuations around Og (g: Gravitational acceleration). It has been found that this irregularity has been mainly caused by the disturbances of the wheel speed signals due to the bouncing of the wheels and the vibrations of the chassis on a rough road surface.
Generally, some chassis control systems, if controlled the same way on rough roads as on good roads, do not operate as well as they might, or should better be differently adjusted for rough and good roads to attain higher stability, maneuverability or safety.
In skid control systems, for example, operation on a rough road often produces an erroneous wheel speed signal due to the irregular vibrations on the road surface so that a false brake cancel signal is sometimes generated, reducing controllability and lengthening the vehicle braking distance considerably over the correct braking distance under locked conditions. Correspondingly, a vehicle height control system, poses the problem of contact between the body chassis and the surface of a rough road. A suspension control system encounters the problem of a reduced riding quality and maneuverability on a rough road. Further, a rough road causes the power steering control system to operate with a lower maneuverability.
The systems or methods for discriminating the conditions of the road surface so far disclosed include a system in which the road surface is scanned by a photo-electric transducer and the reflected brightness is analyzed to determine the road surface conditions (Japanese Patent Laid-Open No. 77946/82) and a system in which a visible light or infrared light is radiated on the road surface and the reflected light is analyzed to discriminate the road surface conditions (Japanese Patent Laid-Open No. 5900/83). These systems are complicated in construction and high in cost, and therefore are not of high practical value for application to vehicles.